1. Field of the Invention
The present invention relates to an air/fuel ratio control apparatus and apparatus for an internal combustion engine, and an engine control unit for conducting a perturbation control that involves periodically fluctuating (vibrating) a target air/fuel ratio over a predetermined amplitude.
2. Description of the Prior Art
Conventionally, an air/fuel ratio control apparatus of the type mentioned above is known, for example, from Laid-open Japanese Patent Application No. 64-66441. This air/fuel ratio control apparatus comprises an O2 sensor disposed at a location downstream of a catalyst in an exhaust pipe for outputting a detection signal in accordance with the concentration of oxygen in exhaust gases. This air/fuel ratio control apparatus calculates an average of the detection signal of the O2 sensor, and calculates a reference value for a perturbation control in accordance with the calculated average. In the perturbation control, the air/fuel ratio control apparatus adds or does not add a predetermined amplitude step to the reference value to calculate an air/fuel ratio correction coefficient, so that the air/fuel ratio correction coefficient, i.e., the air/fuel ratio repeatedly fluctuates within a predetermined amplitude in a rectangular shape. In the perturbation control, the air/fuel ratio is fluctuated at a frequency which is set to a value in a range of 1 to 4 Hz.
While the conventional air/fuel ratio control apparatus sets the frequency to a value in the range of 1 to 4 Hz at which the air/fuel ratio is fluctuated in the perturbation control, the catalyst does not always purify exhaust gases at a constant percentage when this type of perturbation control is conducted. Specifically, it has been found that the catalyst, when not deteriorated, presents a satisfactory value for the exhaust gas purification percentage irrespective of the air/fuel ratio fluctuating frequency of the perturbation control, whereas the catalyst, when deteriorated, presents a satisfactory value in a frequency range not lower than 3 Hz, more preferably not lower than 5 Hz but a significantly lower value in a frequency range lower than 3 Hz (see FIG. 38). As will be supposed from this fact, the conventional air/fuel ratio control apparatus is likely to cause a lower exhaust gas purification percentage presented by the catalyst and a resulting exacerbation in the characteristic of exhaust gases purified by the catalyst (hereinafter called the “post-catalyst exhaust gas characteristic”), when the catalyst is deteriorated, because the air/fuel ratio fluctuating frequency is set to a value in the range of 1 to 4 Hz irrespective of whether or not the catalyst is deteriorated.